The present invention relates generally to the field of optical signal processing apparatus, and more specifically to a spatial light modulator of improved performance.
Two-dimensional spatial light modulators are devices which allow control of an optical wavefront for processing or imaging operations. These devices, often referred to as light valves in the literature, have potential for application in large screen display systems as well as in optical data processing systems, missile guidance systems and robotic vision systems. They may also be used for performing holographic phase conjugation. Listed below are several articles which describe their construction and operation.
1. "A Fast Silicon Photoconductor-Based Liquid Crystal Light Valve", P. O. Braatz, K. Chow, U. Efron, J. Grinberg and M. J. Little, IEEE International Electron Devices Meeting, pp 540-543, 1979. PA1 2. "Oblique-cut LiN.sub.b O.sub.3 Microchannel Spatial Light Modulator", C. Warde and J. I. Thakara, Optics Letters, Vol. 7, No. 7, July 1982. PA1 3. "A First-Order Model of a Photo-Activated Liquid Crystal Light Valve", J. D. Michaelson, SPIE Vol. 218, Devices and Systems For Optical Signal Processing, 1980. PA1 4. "LiNbO.sub.3 and LiTaO.sub.3 Microchannel Spatial Light Modulators", C. Warde, A. M. Weiss and A. D. Fisher, SPIE Vol 218, Devices and Systems for Optical Signal Processing, 1980. PA1 5. "Silicon Liquid Crystal Light Valves: Status and Issues", U. Efron, P. O. Braatz, M. J. Little, R. N. Schwartz and J. Grinberg, Proc. SPIE Vol. 388, January 1983.
Basically, spatial light modulators comprise a photosensitive semiconductor substrate or wafer, a light blocking layer, a dielectric mirror and an electro-optic crystal (which may be a liquid crystal), arranged in a sandwich-like composite structure, and having a voltage applied thereacross. A control (write) illumination impinges on the face of the photosensitive semiconductor while an output (read) illumination makes a double pass through the electro-optic crystal.
The photosensitive semiconductor responds to intensity variations in the control illumination impinging thereon. In the dark, most of the voltage applied across the composite structure appears across the reverse-biased photodiode. The write beam, however, excites carriers in the silicon, which are driven by the internal field to the Si/electro-optic crystal interface. The voltage across the silicon decreases, while the voltage across the electro-optic crystal increases. The read illumination passes through the electro-optic crystal, is reflected off of the dielectric mirror, and again passes through the electro-optic crystal before emerging from the device. Since the diffraction efficiency of the electro-optic crystal is a function of the voltage applied thereacross, (which is a function of the intensity of the write illumination), optical control of the output (read) illumination is achieved.
One of the problems encountered in the practical implementation of such spatial light modulators is that their writing speed is limited by the intensity of the input light. The operational speed of such devices has been somewhat improved in a device called the microchannel spatial light modulator. It consists of a photocathode and a microchannel plate multiplier in proximity focus with an optional planar acceleration grid and an electro-optic crystal plate. The electro-optic plate carries a high-resistivity dielectric mirror on its side nearest the microchannel plate multiplier. Incoherent or coherent light incident upon the photocathode produce charge which is amplified by the microchannel plate multiplier and is proximity focused onto the mirror surface, to modulate the refractive index of the electro-optic crystal. The image is erased by flooding the photocathode with light to remove electrons from the mirror surface by secondary electron emission. Alternatively, the image is written by secondary electron emission and erased by adding electrons to the electro-optic crystal. Such a device, which exploits the secondary electron emission characteristics of the dielectric mirror, is described in the article entitled "Oblique-cut LiNbO.sub.3 Microchannel Spatial Light Modulator", supra.
The additional amplification achieved by secondary emission at the surface of the dielectric mirror improves somewhat the speed of operation of such devices, perhaps by an order of magnitude or less. However, further improvements in operational speed and other performance characteristics cannot be obtained once the saturation current of the microchannel plate multiplier is reached. Furthermore, at high current outputs, the temperature of such microchannel plate multipliers increases to a value where their operational lifetime is drastically reduced.